1. Field of the Invention
The invention relates to the measurement of voltage and current in a high-voltage power transmission line from a position which is electrically isolated from the line by electro-optical means.
2. Description of the Prior Art
U.S. Pat. No. 3,466,541 (Bernard et al.) teaches the use of a crystal of KDP as a Pockels cell in an electrical (i.e. voltage-gradient) field to measure the intensity of the voltage on a high-voltage line. U.S. Pat. No. 3,605,013 (Yoshikawa et al.) teaches the use of a particular glass fiber as a Faraday cell for measuring a magnetic (i.e. Gaussian) field about a high voltage line. U.S. Pat. No. 3,675,125 (Jaecklin) teaches the use of a Pockels cell and a Faraday cell in combination to measure the power in a high-voltage line. U.S. Pat. No. 3,597,683 (Saito et al.) teaches the use of a Faraday effect element to measure the intensity of a magnetic field in the vicinity of a high-voltage line.
None of these references teaches the use of any temperature compensation to compensate for temperature variations in the sensing elements. There is a very large temperature coefficient in the KDP-type elements, which are the best Pockels-effect sensors. A change in temperature from 45.degree. F. (7.degree. C.) to 65.degree. F. (18.degree. C.) would cause an estimated 6% error in readings from a KD*P crystal. The Faraday-effect sensors are less sensitive to temperature variations, but they can also benefit from temperature compensation.
Only Saito teaches a device which can be physically and electrically isolated from the line, which is a practical necessity when dealing with lines in the range of 500 kV. None of the references teaches the use of the devices with filters to measure transients, which are such short-term effects that temperature compensation can be ignored.